1. Field of the Invention
The present invention relates to a steel sheet for a heat-shrink band.
2. Description of the Related Art
In a color cathode ray tube (CRT), in order to have the electrons emitted from an electron gun strike the fluophors on an inner surface of a panel portion of the CRT so as to cause them to emit the desired light, the interior of the tube comprising the panel portion and a funnel portion joined to this is evacuated to a high vacuum of about 1.0.times.10.sup.-7 Torr so as to prevent the flight of the electrons from being hindered by any gas remaining in the inside. When the tube is evacuated, the panel surface is deformed to a concave shape in comparison with the state before evacuating the interior of the tube due to the air pressure.
In this state, the positions of the fluophors provided inside the panel portion shift in comparison with the state before the evacuation of the interior of the tube, therefore the landing points of the electron beams relative to the fluophor panel shift, so color deviation is caused. Further, if the panel surface is deformed to the concave state, there is a risk of implosion.
In order to prevent this, a heat-shrink band obtained by shaping a sheet of steel into a band is provided around the periphery of the panel portion. The inner circumferential length of the band at an ambient temperature becomes slightly smaller than the length of the periphery of the panel portion.
In order to fit this heat-shrink band around the periphery of the panel portion, the band is first heated to about 500.degree. C. to cause it to expand, then the band is fit over the periphery of the panel portion and simultaneously rapidly cooled by means of blowing air etc. This rapid cooling causes the heat-shrink band to shrink. The deformation of the panel surface due to the air pressure is corrected by the resultant tension. As this heat-shrink band, conventionally use is made of a sheet of steel material having a stress of yielding point of about 24 kg/mm.sup.2 (elongation of 30%) in view of the balance of the formability and tension.
However, a heat-shrink band has to provide a tension large enough to correct the deformation of the panel surface due to the air pressure as described above. With conventional materials, there has been the problem that they are heavy in weight--over 700 g even for a 21 inch cathode ray tube. If it is attempted to use a high tensile strength steel sheet having a high stress of yielding point as the heat-shrink band in order to solve such a problem, there is the problem in that with sheets of steel, there is the generally the tendency that the higher the stress of yielding point rises, the smaller the elongation becomes (for example about 10%), so the formability when shaping the heat-shrink band by the press process or the like becomes remarkably poor and cracks or splits will occur in the bent part.
In the above description, the heat-shrink steel band for the CRT was discussed, the above disadvantages can be applied to other objects.